It is well known that when impure iron (e.g. cast iron) is in contact with water, dissolved oxygen or other strong oxidants (or acids) it rusts. Iron metal is relatively unaffected by pure water or by dry oxygen. As with other metals, like aluminum, a tightly adhering oxide coating, a passivation layer, protects the bulk iron from further oxidation. The conversion of the passivating ferrous oxide layer to rust results from the combined action of two agents, usually oxygen and water. However, other degrading solutions such as those of sulfur dioxide and/or carbon dioxide in water create corrosive conditions where iron hydroxide species are formed. Unlike ferrous oxides, the hydroxides do not adhere to the bulk metal; as they form and flake off, fresh iron is exposed and the cycle is repeated.
The rusting of iron is an electrochemical process that begins with the transfer of electrons from iron to oxygen. The rate of corrosion is affected by water and accelerated by electrolytes, such as those from salts present. The key reaction is the reduction of oxygen (i.e. molecular oxygen O2). Therefore one approach to reducing and/or preventing corrosion of metals, particularly those containing iron, in contact with water and oxygen is to “scavenge” or bind up the oxygen before it has a chance to oxidize the iron.
While mechanical deaeration of water is an important step, in many systems and processes, mechanical deaeration is followed by chemical deaeration in order to remove (bind up or otherwise scavenge) the last traces of dissolved oxygen. Where mechanical deaeration is not employed, chemical deaeration must be used for the removal of the entire oxygen content of the aqueous system.
Sodium sulfite and sodium bisulfite are chemical agents commonly used for scavenging oxygen, in non-limiting instances, oilfield production systems, such as from produced water systems or water injection systems to reduce the potential for oxygen-induced corrosion. However in the presence of glycols, the oxygen removal reactions become challenged by glycols that act to terminate the chain reactions and prevent oxygen removal. Thus, glycols interfere with the use of sulfites as oxygen scavengers.
It would be advantageous if new oxygen scavenger compositions were discovered or developed that could be used in aqueous systems where glycol is present.